This invention relates generally to the field of rigid disc drives, and more particularly, but not by way of limitation, to a glide test head assembly for use in testing magnetic disc recording media surface characteristics.
Disc drives of the type known as "Winchester" disc drives or hard disc drives are well known in the industry. Such disc drives record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent to the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator housing opposite to the coil, the actuator housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator housing rotates, the heads are moved radially across the data tracks along an arcuate path.
As the physical size of disc drives has decreased historically, the physical size of many of the disc drive components has also decreased to accommodate this size reduction. Similarly, the density of the data recorded on the magnetic media has been greatly increased. In order to accomplish this increase in data density, significant improvements in both the recording heads and recording media have been made.
For instance, the first rigid disc drives used in personal computers had a data capacity of only 10 megabytes, and were in the format commonly referred to in the industry as the "full height, 51/4" format. Disc drives of the current generation typically have a data capacity of over a gigabyte (and frequently several gigabytes) in a 31/2" package which is only one fourth the size of the full height, 51/4" format or less. Even smaller standard physical disc drive package formats, such as 21/2" and 1.8", have been established. In order for these smaller envelope standards to gain market acceptance, even greater recording densities must be achieved.
The recording heads used in disc drives have evolved from monolithic inductive heads to composite inductive heads (without and with metal-in-gap technology) to thin-film heads fabricated using semi-conductor deposition techniques to the current generation of thin-film heads incorporating inductive write and magneto-resistive (MR) read elements. This technology path was necessitated by the need to continuously reduce the size of the gap in the head used to record and recover data, since such a gap size reduction was needed to reduce the size of the individual bit domain and allow greater recording density.
Since the reduction in gap size also meant that the head had to be closer to the recording medium, the quest for increased data density also lead to a parallel evolution in the technology of the recording medium. The earliest Winchester disc drives included discs coated with "particulate" recording layers. That is, small particles of ferrous oxide were suspended in a non-magnetic adhesive and applied to the disc substrate. With such discs, the size of the magnetic domain required to record a flux transition was clearly limited by the average size of the oxide particles and how closely these oxide particles were spaced within the adhesive matrix. The smoothness and flatness of the disc surface was also similarly limited. However, since the size of contemporary head gaps allowed data recording and retrieval with a head flying height of twelve microinches (0.000012 inches, 12.mu.") or greater, the surface characteristics of the discs were adequate for the times.
Disc drives of the current generation incorporate heads that fly at nominal heights of only about 2.0.mu.", and products currently under development will reduce this flying height to 1.5.mu." or less. Obviously, with nominal flying heights in this range, the surface characteristics of the disc medium must be much more closely controlled than was the case only a short time ago.
In current disc drive manufacturing environments, it is common to subject each disc to component level testing before it is assembled into a disc drive. One type of disc test is referred to as a "glide" test, which is used as a go/no-go test for surface defects or asperities, or excessive surface roughness. A glide test typically employs a precision spin stand and a specially configured glide test head including a piezo-electric sensing element, usually comprised of lead-zirconium-titanate (PbZrTi.sub.3), also commonly known as a "pzt glide test head". The glide test is performed with the pzt glide test head flown at approximately half the flying height at which the operational read/write head will fly in the finished disc drive product. For instance, if the disc being glide tested is intended for inclusion in a disc drive in which the operational heads will fly at 2.0.mu.", the glide test will typically be performed with the pzt glide test head flying at 1.0.mu.". If the glide test is completed without contact between the pzt glide test head and any surface defects, then the disc is passed on the assumption that there will be no contact between the operational heads and the discs during normal operation with a nominal head flying height twice that of the pzt glide test head flying height.
A variant of the glide test, often used by disc media manufacturers, is sometimes referred to as a "glide avalanche" or GA test. In GA testing, a pzt glide test head is first flown at a greater than normal flying height above the disc surface. This initial increased flying height is commonly achieved by rotating the disc under test at a greater than normal speed, thus increasing the linear velocity between the disc and the test head, and increasing the strength and thickness of the air bearing supporting the test head above the disc surface.
The rotational speed of the disc under test is then gradually reduced until contact between the test head and disc occurs, at which point the current flying height is recorded. Correlation of a series of such test sequences at varying radii on the disc can be used by the disc media manufacturer as an indication of overall disc surface characteristics.
It is also common practice in the industry to provide a textured "landing zone" on the disc surface, on which the read/write head of the disc drive will come to rest during "power-off" or "sleep" conditions. Since the glide avalanche test simulates the loss of power to rotate the disc, the glide avalanche test is also frequently used by design engineers developing textured landing zones to study the landing characteristics of head assemblies on various types of landing zone textures.
The read/write head assemblies incorporated in disc drive products are commonly designed to provide rapid take-off of the head assemblies as the discs accelerate from stopped to operational speed to minimize frictionally-induced wear, and typical pzt-glide test heads also include air bearing structures with the same rapid take-off characteristics.
One technique frequently used to lower the linear velocity between discs and head assemblies at which head take-off occurs is to provide the air bearing surfaces of the head assemblies with a positive crown, or slightly convex surface. As is known to those of skill in the art, a positive crown on the air bearing surfaces of a head assembly causes the hydrodynamic pressure between the heads and the discs to increase rapidly with the increase in linear velocity between the heads and discs, and thus enables the head assemblies to begin to fly at a much lower linear velocity than would heads with perfectly planar air bearing surfaces.
It is also common practice in the industry to utilize such positive crowns on the air bearing surfaces of glide test heads.
However, when such glide test heads are used for the glide avalanche test described above, the low linear velocity necessary to bring about contact between the test head and the disc may also result in instability of the flying attitude of the test head at the time of contact, and subsequent ambiguity in the validity of the glide avalanche test results.
A need exists, therefore, for a glide avalanche test head assembly which is capable of stable flight at lower flying heights to enable reliable glide avalanche testing on discs including the extremely smooth surfaces of the current generation of disc media products.